The influences of central wavelength of the seed pulse and gain fiber length of the amplifier on the characteristics of broadband spectra are systematically investigated based on nonlinear ytterbium-doped fiber amplifiers. The dissipative solitons from the nonlinear amplifier are obtained by using the nonlinear polarization rotation mode-locking technique. The best flat wide spectrum with wavelength range of 1040-1600 nm is obtained when the center wavelength of the seed pulse is 1041 nm and the gain fiber length of the amplifier is 8 m. The flatness is about 10 dB, and the wide spectral flatness is less than 1.5 dB from 1040 to 1250 nm.